1. Field of the Invention
The invention relates to thermoplastically processible elastomers based on comb polymers with improved optical properties, in particular high transparency.
2. Discussion of the Background
The polymers derived from monomeric "soft" acrylate esters can extend over the entire range from soft elastomers to hard, non-film-forming elastics, depending on the content of "hard" comonomers in the polymer (see Kirk-Othmer, 1978, "Encyclopedia of Chemical Technology", 3rd Ed Vol 1 pub. J. Wiley, pp. 386-408). The typical elastomeric polyacrylates are distinguished by good oil resistance and relatively high thermal stability. However, their mechanical properties at cold temperatures and elevated temperatures leave something to be desired.
DE-A 39 02 653 and U.S. Pat. No. 5,155,172 describe elastomers based on acrylates containing 40-100 wt. % of one or more copolymers, with molecular weights greater than 50,000 Dalton, comprised of
acrylate monomers in the amount of 50-95 wt. %, and PA1 macromonomers comprised of a vinyl group and a polyvinyl unit covalently bonded thereto, selected from the group consisting of the acrylates, the methacrylates, and the (possibly substituted)styrenes. PA1 (A) 5-50 wt. %, preferably 20-40 wt. %, of a macromonomer (MM) comprised of PA1 methyl methacrylate or PA1 copolymers of methyl methacrylate with comonomers chosen from the group of other (meth)acrylic acid esters, preferably of C.sub.1 -C.sub.12 alcohols, and (possibly substituted)styrenes, containing a terminal unit with a radically polymerizable group having a glass temperature Tg of at least 60.degree. C. a molecular weight in the range 500-100,000 Dalton, and an index of refraction in the range n.sub.D =1.48-1.50; PA1 (B) 5-90 wt. %, preferably 50-80 wt. % (based on the total weight of the monomers in CP) of monomers or monomer mixtures of formula I: ##STR2## where R.sub.1 represents C.sub.1 -C.sub.12 alkyl group; and (C) 5-50 wt. %, preferably 10-30 wt. %, of a phenyl group-containing radically polymerizable monomer (PHM), wherein the monomer groups (B) and (C) are preferably in a weight ratio in the range 10:1 to 1:2. Preferably R.sub.1 in formula I represents a C.sub.4 -alkyl group, e.g. n-butyl. PA1 (1) the monomers of formula III: ##STR3## where R.sub.2 represents hydrogen or methyl; PA1 R.sub.3 represents a phenyl group, possibly substituted; and PA1 n represents 0, 1, 2, or 3; and PA1 (2) the monomers of formula IV: ##STR4## where R.sub.4 represents hydrogen of a C.sub.1 -C.sub.4 -alkyl group and PA1 R.sub.5 represents hydrogen or a methyl group.
According to DE-A 39 02 653, with appropriate limits on the molecular weight, the resulting comb polymers are thermoplastically processible elastomers (TPEs). Comb polymers with, e.g. a polybutylacrylate main chain and polymethylmethacrylate (PMMA) side chains are as rule highly transparent when the PMMA side chains are strongly bound to the butyl acrylate main chain, even though there is an appreciable difference between the index of refraction of the PMMA hard phase (n.sub.D.sup.20 of PMMA=1.492) and that of the polybutyl acrylate phase (n.sub.D.sup.20 of PBuA=1.466). It is possible to explain this result based on the assumption of very small PMMA domains (see Platzer, N. A. J., Ed., 1971, "Multicomponent polymer systems", Adv in Chem Ser , 99, pub. Am. Chem. Soc., pp. 12-13).
In the interest of maximally unconstrained processing and application of thermoplastically processible elastomers, it is necessary to have systems with constant optical behavior which are not appreciably affected by prior treatment and the processing conditions. Accordingly, the underlying problem of the invention is to devise TPEs which have the desired optical properties, particularly transparency, independently of phase structure or domain size, even in mixtures with PMMA. It has been discovered, in connection with the invention, that, starting with the comb polymers of DE-A 39 02 653 (U.S. Pat. No. 5,219,931)and EP-A 0381065, (U.S. Pat. No. 5,155,172) one may obtain thermoplastically processible elastomers (TPEs) with improved optical properties by copolymerizing with phenyl group-containing monomers.